Abstract The three-dimensional structure of an accelerating flame front makes it very difficult to visualize. Most experimental studies that provided a visualization of flame acceleration and deflagration-to-detonation transition (DDT) events were done in rectangular tubes using the shadow technique. Qualitative information obtained by these optical methods has been two-dimensional and line-of-sight integrated, so local peculiarities and trends of the phenomena may not be spatially resolved in the third dimension. Here, we present a high-speed imaging analysis (self-luminous stereoscopic photography) of the final stage of fast flame propagation, the formation of new autoignition kernels and the onset of localized explosions in a long smooth transparent cylindrical tube. We investigated the evolution of the accelerating flame shape along the tube and found that before transition to detonation the flame shape is quite stable and looks like a “paper cone” or “waffle cone”, instead of the usual shape that is referred to as a “tulip flame”. Simultaneous images captured by two high-speed cameras positioned 90° apart allowed us to determine the location of the auto-ignition kernels and the explosion points in reaction gas volume behind the leading shock wave. Both the horizontal position with respect to the leading edge of the flame and the position across the tube cross-section were determined. We observed the four typical scenarios for the onset of detonation in front of the accelerating flame where explosion occurs in the boundary layer to be: (1) between the secondary reaction fronts that emerge from the auto-ignition kernels; (2) between the main flame front and the secondary reaction fronts that emerge from the auto-ignition kernels; (3) at the leading tip of the secondary reaction fronts; (4) at the tip of the main turbulent flame front. We determined the local gas parameters between the flame front and the leading shock wave directly before the onset of detonation. The flow stagnation in the boundary layer near the tube wall leads to a significant local temperature increase and strongly reduces the ignition delay time of the mixture. Simple kinetic calculations show that for our test conditions, a temperature rise of approximately 275 K results in an induction time reduction by 15 times.

中文翻译：

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使用高速立体成像分析圆柱形管中火焰加速的最后阶段和爆炸的开始

摘要 加速火焰锋面的三维结构使其很难可视化。大多数提供火焰加速和爆燃到爆轰转变 (DDT) 事件可视化的实验研究都是在矩形管中使用阴影技术完成的。通过这些光学方法获得的定性信息已经进行了二维和视线整合，因此现象的局部特征和趋势可能无法在三维空间上解析。在这里，我们展示了快速火焰传播最后阶段的高速成像分析（自发光立体摄影）、新自燃核的形成以及长光滑透明圆柱管中局部爆炸的开始。我们研究了沿管的加速火焰形状的演变，发现在过渡到爆炸之前，火焰形状非常稳定，看起来像一个“纸锥”或“华夫饼锥”，而不是通常被称为“郁金香火焰”。由两个相距 90° 的高速摄像机同时捕获的图像使我们能够确定自燃内核的位置以及领先冲击波后面反应气体体积中的爆炸点。确定了相对于火焰前缘的水平位置和穿过管横截面的位置。我们观察到在边界层发生爆炸的加速火焰前开始爆炸的四种典型情况是：(1) 在自燃核中出现的二次反应前沿之间；(2) 主火焰锋与自燃核出现的次要反应锋之间；(3) 在次级反应锋的前沿；(4) 在主要湍流火焰前沿的尖端。我们直接在爆炸开始之前确定了火焰前沿和前导冲击波之间的局部气体参数。管壁附近边界层的流动停滞导致局部温度显着升高，并大大降低了混合物的点火延迟时间。简单的动力学计算表明，对于我们的测试条件，大约 275 K 的温度升高导致诱导时间减少 15 倍。(2) 主火焰锋与自燃核出现的次要反应锋之间；(3) 在次级反应锋的前沿；(4) 在主要湍流火焰前沿的尖端。我们直接在爆炸开始之前确定了火焰前沿和前导冲击波之间的局部气体参数。管壁附近边界层的流动停滞导致局部温度显着升高，并大大降低了混合物的点火延迟时间。简单的动力学计算表明，对于我们的测试条件，大约 275 K 的温度升高导致诱导时间减少 15 倍。(2) 主火焰锋与自燃核出现的次要反应锋之间；(3) 在次级反应锋的前沿；(4) 在主要湍流火焰前沿的尖端。我们直接在爆炸开始之前确定了火焰前沿和前导冲击波之间的局部气体参数。管壁附近边界层的流动停滞导致局部温度显着升高，并大大降低了混合物的点火延迟时间。简单的动力学计算表明，对于我们的测试条件，大约 275 K 的温度升高导致诱导时间减少 15 倍。我们直接在爆炸开始之前确定了火焰前沿和前导冲击波之间的局部气体参数。管壁附近边界层的流动停滞导致局部温度显着升高，并大大降低了混合物的点火延迟时间。简单的动力学计算表明，对于我们的测试条件，大约 275 K 的温度升高导致诱导时间减少 15 倍。我们直接在爆炸开始之前确定了火焰前沿和前导冲击波之间的局部气体参数。管壁附近边界层的流动停滞导致局部温度显着升高，并大大降低了混合物的点火延迟时间。简单的动力学计算表明，对于我们的测试条件，大约 275 K 的温度升高导致诱导时间减少 15 倍。